1. Field of the Invention
The present invention relates to an electrode discharge machining apparatus in which a minute gap between a wire electrode and a workpiece is filled with machining liquid, a discharge is caused in the gap, whereby the workpiece is machined.
2. Description of the Related Art
FIG. 18 is a diagram showing a first conventional wire electrode discharge machining (EDM) apparatus (referred to as a prior art 1). In FIG. 18, reference numeral 1 is a bed as a base of the wire EDM apparatus; 2 is a table put on the bed 1; 3 is a saddle mounted on the bed 1 mounted on the table 2 by a guide such that it is horizontally movable with respect to the table 2; 4 is a column mounted on the saddle 3 by a guide such that it is movable to the front and back directions with respect to the table 2; 5 is a beam member coupled with the front side of the column 4 by a guide vertically movable with respect to the table 2; 6 is an upper wire guide fixed to the beam member 5; 7 is a lower arm fixed to the column 4; 8 is a lower wire guide fixed to the tip of the lower arm 7; 9 is a workpiece 9 fixedly mounted on the table 2; 10 is a machining bath put on the bed 1; 11 is a wire electrode supported by the upper and lower wire guides 6 and 7; 12 is a machining liquid supplying means; 13 is a machining liquid cooling means; 14 is a temperature sensor for the machining liquid cooling means 13; and 15 is insulating, machining liquid, which is supplied from the machining liquid supplying means 12 to the machining bath 10 by way of the machining liquid cooling means 13. The machining liquid 15, soiled through a machining process, is gathered by the machining liquid supplying means 12 and filtered, and supplied again to the machining bath 10. The machining liquid cooling means 13 controls a temperature value of the machining liquid 15 so that it is equal to the sum of a temperature value detected by the temperature sensor 14 and an offset value. To machine the workpiece, a pulse voltage is applied between the wire electrode 11 and the workpiece 9, from a power source, not shown. Discharge takes place in the fine gap between the wire electrode 11 and the workpiece 9, to thereby effect the machining of the workpiece.
FIG. 19 is a diagram showing a second conventional wire electrode discharge machining apparatus (prior art 2), disclosed in Japanese Patent Laid-Open Publication No. Sho. 62-264830.
The construction of the wire EDM apparatus shown in FIG. 19 will be described hereunder. In the figure, reference numeral 46 is a bed; 47 is a column; 48 is an upper arm; 49 is a lower arm; 50 is a wire electrode; 51 is an electrode reel; 52 is a take-up roll; 53 is an upper guide; 54 is a lower guide; and 55 is a UV table for positioning the upper guide 53 when it is driven by a control unit, not shown. 56 is an XY table driven to move in two directions, orthogonal to each other, when it is driven by the control unit, not shown. 57 is a workpiece; 58 is workpiece supporting means; 59 is a shielding member; 60 is a bellow means; and 61 is an air inlet.
In FIG. 19, the shielding member 59 forms a space, substantially closed, in at least a major structure. For the UV table 55 and the XY table 56, the bellow means 60 is provided in a part of the shielding member 59 and a plural number of air inlets 61 and an air outlet, not shown, are provided therein. Gas is introduced into the space formed by the shielding member 59, through the air inlets 61, whereby the space is kept at fixed temperature and humidity values. The temperature and humidity values of the gas introduced through the air inlets 61 are adjusted corresponding to the temperature and humidity values in the space.
FIG. 20 is a diagram showing a third conventional wire electrode discharge machining apparatus (prior art 3), disclosed in Japanese Patent Laid-Open Publication No. Sho. 63-99128.
The construction of the wire EDM apparatus shown in FIG. 20 will be described hereunder. In the figure, reference numeral 62 is a tank for storing machining liquid; and 64 is a cooler for cooling machining liquid 63. The machining liquid 63, after cooled by the cooler, is led through a pulse power source unit 66 to a first passage A by a pump 65. The first passage A leads the machining liquid to between the electrodes, through an upper machining liquid ejecting nozzle 67 and a lower machining liquid ejecting nozzle 68. The machining liquid 63 is supplied, by the pump 65, to a second passage B, through upper and lower arms 69 and 70, and a machining bed 72 on which a workpiece 71 is located. Two passages, the first and second passages A and B, are provided for the cooled machining liquid 63 in order to improve a cooling efficiency. In the first passage A, the machining liquid is first used to cool the pulse power source unit 66, and the heated liquid is fed to between the electrodes. The second passage B is used for cooling the mechanism.
FIG. 21 is a diagram showing a fourth conventional wire electrode discharge machining apparatus (prior art 4), disclosed in Japanese Patent Laid-Open Publication No. Sho. 63-179024.
The construction of the wire EDM apparatus shown in FIG. 21 will be described hereunder. In the figure, reference numeral 73 is a workpiece; 74 is a surface plate; 75 is an upper wire guide; 76 is a lower wire guide; 77 is a wire electrode; 78 is a lower arm; 79 is a column; 80 is a pipe; 81 is a motor driven fan; and 82 is air.
The wire electrode discharge machining apparatus machines the workpiece 73 in a desired shape by the wire electrode 77, while blowing machining liquid against a machining part of the workpiece 73. In this EDM apparatus, a holding means of the lower wire guide 76 for holding the wire electrode 77 is hollowed. The EDM apparatus is provided with means for feeding cooled fluid into the hollow of the holding means.
FIG. 22 is a diagram showing a fifth conventional wire electrode discharge machining apparatus (prior art 5), disclosed in Japanese Patent Laid-Open Publication No. Sho. 61-86130.
The construction of the wire EDM apparatus shown in FIG. 22 will be described hereunder. In the figure, reference numeral 83 is a main body of the wire electrode discharge machining apparatus; 84 is a control unit including an NC device and a machining power source; 85 is a machining liquid supplying device; 86 is a cable for connecting the control unit 84 to the machining apparatus body 83; and 87 is a cable for connecting the machining apparatus body 83 to the machining liquid supplying device 85. Numeral 88 is a machining liquid exhausting pipe for discharging machining liquid from the machining apparatus body 83 to the machining liquid supplying device 85. Numeral 89 is a filter for filtering the machining liquid in the bath. 90 is an outside temperature sensing device for measuring temperature outside the machining apparatus body 83. 91 is a machining liquid temperature sensing device for measuring temperature of the machining liquid within the bath. 92 is a preheater for preheating the machining liquid of the bath up to a temperature set in advance by the control unit 84. 93, 94 and 95 are cables for connecting the control unit 84 to the outside temperature sensing device 90, machining liquid temperature sensing device 91, and preheater 92.
Problems of the wire electrode discharge machining apparatus of the prior art 1 follows. In the EDM apparatus, the lower arm extended from the column is put in the machining liquid and the beam member also extended from the column is put in the air. A temperature difference appears between the beam member and the lower arm because there is a temperature difference between the machining liquid and the discharge machining apparatus, a difference between the time constants of their temperature variations of them and outside air. The thermal expansions of the lower arm and the beam member displace those members from their correct positions. As a result, the upper wire guide is positionally shifted relative to the lower wire guide, which are held at the tips of the upper and lower arms. This leads to the deterioration of the straightness of the wire electrode and the machining accuracy of the discharge machining apparatus.
To secure a highly accurate machining, the wire EDM apparatus may be installed in a thermostatic room whose temperature is controllable. It is inevitable, however, that a person comes in and goes out of the thermostatic room. In other words, thermal disturbances are inevitably generated. FIG. 23 is a graph showing a variation of room temperature around the wire EDM apparatus which is placed in a thermostatic room. As seen from the graph, room temperature is varied approximately 1.5.degree. C. for a day. The temperature increase corresponds to 20 .mu.m per 1 m of the thermal expansion of the member whose thermal expansion coefficient is 11.8.times.10-61/.degree. C. Since a temperature sensor put on the cast structure directly contacts with the gas in the constant temperature room, the room temperature variation affects the temperature values sensed by the sensor. Therefore, the machining liquid temperature varies with a variation of the room temperature. The temperature variation of the cast structure does not instantaneously respond to the room temperature variation. Therefore, temperature of a portion of the cast structure directly contacting with the machining liquid is different from temperature of the remaining portion not contacting with the machining liquid. The result is that a thermal expansion difference is created between the upper and lower wire guides, and the machining accuracy of the wire EDM apparatus is degraded.
The problems of the conventional wire EDM apparatus of the prior art 2 follows. In the EDM apparatus, gas is introduced into the space encompassed by the shielding member that surrounds a major structure of the wire EDM apparatus, to thereby control temperature of the gas around the wire EDM apparatus located in the space enclosed by the shielding member. In order that temperature of the major structure of the EDM apparatus of large heat capacity and temperature of the gas within the shielding member are kept constant, the air conditioner of large capacity and high performance is required. It is difficult to manufacture the products or the EDM apparatuses at practically acceptable prices, however.
A large room is required for installing the EDM apparatus. Therefore, when the wire EDM apparatus of the prior art 2 occupies a large volume of space in the limited space within the constant temperature room. In an extreme case, the installing of the EDM apparatus in the room is impossible or the expansion of the constant temperature room is required.
Further, a large amount of heat is generated by the heat exchanger of the air conditioner of the EDM apparatus. Under this condition, if the wire EDM apparatuses of the prior art 1 and 2 are installed close to each other, a thermal deformation of the prior art 1 is increased, so that the machining accuracy of the prior art 2 is deteriorated.
Where temperature inside the shielding member is directly controlled by introducing gas into the inside of the shielding member, a temperature difference is created between a portion of the major structure of the EDM apparatus where it directly comes in contact with the temperature controlled gas and the remaining portion not directly contacting with the same, and between the portions of the major structure close to and far from the gas inlets. The temperature difference leads to thermal expansion differences of those portions, and degradation of the machining accuracy of the wire EDM apparatus.
The conventional EDM apparatus of the prior art 3 is a wire cut electrode discharge machining apparatus. This type of the apparatus carries out the discharge machining while applying voltage between the wire electrode and the workpiece. The EDM apparatus includes two machining liquid passages, first and second passages. In the first passage, cooled machining liquid is fed from the tank to between the wire electrode and the workpiece, through the power source unit and the upper and lower nozzles. In the second passage, the machining liquid is fed to a region at and near the workpiece, through the upper and lower arms and the machining bed on which the workpiece is located. Therefore, a temperature difference is created between the machining liquid flowing through the upper and lower arms and the machining liquid that is fed from the tank to between the wire electrode and the workpiece, through the power source unit and the upper and lower nozzles. The temperature difference brings about the thermal expansion difference of the mechanical parts and components in the apparatus, and machining accuracy degradation.
Where the machine body is cooled by the cooled machining liquid, some portions of the machine body are cooled and some portions are not cooled. Therefore, a temperature distribution is not uniform in the wire electrode discharge machining apparatus. The result is that local deformations appear in the wire electrode discharge machining apparatus, and thermal expansions of the upper and lower arms, and the machining bed are different from one another. To make uniform the temperature distribution of the machine body, it is necessary to cool the upper and lower arms, and further whole machine structure including the machining bed. The result is an increase of the cooling capacity of the cooler and an increase of the complexity of the cooler structure, and therefore makes it difficult to manufacture the products of practically acceptable costs.
The wire electrode discharge machining apparatus of the prior art 4 machines the workpiece in a desired shape by the wire electrode, while blowing machining liquid against a machining part of the workpiece. In this EDM apparatus, a holding means of the lower wire guide 76 for holding the wire electrode 77 is hollowed. Cooled fluid is fed into the hollow of the holding means. If the machining liquid whose temperature is controlled in accordance with outside temperature, variations of temperature of the main body of the discharge machining apparatus and outside temperature will have different time constants. As a result, a thermal expansion difference will be created between the wire guide and the mechanical structure for holding the work, and the machining accuracy of the discharge machining apparatus will be deteriorated.
In the wire electrode discharge machining apparatus of the prior art 5, the machining liquid is preheated in accordance with an increased temperature value of the machining liquid currently used for machining, which is measured or calculated, and an outside temperature value measured by a thermal sensor device. The preheated machining liquid is circulated through the main body of the discharge machining apparatus. To set the respective points of the whole apparatus at the temperature value of the preheated machining liquid, it is necessary to arrange the passage for circulating the machining liquid throughout the apparatus. In the discharge machining apparatus, the liquid circulating passage is, of necessity, long. The long passage results in a complex structure of the discharge machining apparatus, and an increase of cost to manufacture. The discharge machining apparatus where temperature of the machining liquid follows a variation of outside temperature is disadvantageous in that if the machining liquid is circulated through a part of the discharge machining apparatus, a time constant of a temperature variation at a portion of the apparatus where it comes in contact with the machining liquid becomes different from temperature variation time constants at the remaining portions thereof where those are not in contact with the machining liquid. The result is to increase a deviation in a temperature distribution of the apparatus. To avoid such a thermal deviation, it is necessary to arrange the machining liquid passage throughout the apparatus. This necessitates a long machining liquid passage and high cost to manufacture.